The Fudgets system is a toolkit for developing graphical applications in the lazy functional programming language Haskell. In this paper we develop an operational semantics for a subset of this system, inspired by ideas from concurrency theory. A semantic theory based on bisimulation is defined and shown to be a congruence. We consider two applications of this theory: firstly, some equational rules useful for reasoning about Fudget programs are verified; secondly, we show how the operational semantics can be used to check the correctness of implementations of the Fudgets system. 1 Introduction The Fudgets system [11] is one of a number of systems for building graphical applications in lazy functional programming languages such as Haskell [24]. A program in the Fudgets system is composed of a collection of fudgets, which correspond closely to concurrent processes in a process network. Fudgets can communicate with one another and also with the window system, and correspond to graphical...